The proposed study will be composed of multiple N -of 1 studies performed in a randomized, double-blind, placebo-controlled multiple (4) crossover fashion. Twelve PD patients (thus 12 N-of-1 trials) with levodopa-induced dyskinesia will complete 4 treatment phases; one phase at each of the 3 doses of famotidine and one of placebo. The treatment doses will be: famotidine 40mg/day, 80mg/day, and 120mg/day. The phases will occur in a random order, but each subject will receive each of the treatment doses during the course of the study. After each phase of treatment (14 days), there will be a washout period (7 days, over 10 half-lives of famotidine) followed by a crossover. The primary outcome will be the change in Unified Dyskinesia Rating Scale (UDysRS) between placebo and famotidine at the end of each treatment phase and secondary outcomes will be Movement Disorder Society Unified Parkinson Disease Rating Scale (MDS-UPDRS) parts III and IV, Clinical Global Impression (CGI), Lang-Fahn activities of daily living dyskinesia scale (LFADLDS) and assessment of adverse effects.